Sputtering, as a means to deposit thin films of a desired material on a substrate, is an old art which has become very important in the manufacture of semiconductor devices such as integrated circuits. In a sputtering system material to be deposited on the substrate is removed from a sputter target by bombardment of the target with ions. However, ion bombardment not only causes atoms or molecules of the target material to be ejected from the target, it also imparts thermal energy to the target.
Normally, a sputtering system comprises a sputter source, a vacuum chamber, and means for positioning and holding the substrate in proximity to the sputter source. A modern sputter source normally comprises a target from which material is to be sputtered, means for holding the target in place, means for creating a plasma in the vicinity of the target, means for causing ions in the plasma to bombard the target, and means for cooling the target to prevent overheating.
Various means have been used in the past for holding sputter targets in place in sputter sources. Such holding means must ensure that the target maintains good thermal contact with the cooling means. In a vacuum environment this generally requires good physical contact. On the other hand, sputter targets must be replaced from time to time as the target material is removed and the target is eroded away. Moreover, in certain research applications there is the need for frequent target changes so that experiments can proceed with different materials.
The time required to replace a sputter target and the method of replacement are often significant considerations to users of sputtering systems. The semiconductor manufacturing industry is particularly sensitive to equipment "down-time", (i.e., the time a piece of production equipment is unavailable) because of the disruptive and costly effects it can have on an entire semiconductor fabrication line. Similarly, the method of target replacement should minimize the introduction of foreign material into the sputtering system's vacuum chamber to avoid possible contamination problems with the substrate.
In some prior art sputter sources, particularly in sputter sources designed to use planar targets, the sputter target is physically soldered to a cooling plate. While this ensures good thermal contact, it makes replacement of the target a difficult and time-consuming problem. In other designs, targets are physically bolted to a cooling means. While this makes target replacement somewhat easier, the thermal contact is not as good. There is a class of sputter sources having annular targets in which target cooling is accomplished by thermal contact between an outer rim of the target and a peripheral cooling wall. In such sources, good thermal contact is maintained between the target rim and the cooling wall by thermal expansion of the target. Examples of such sources are shown in U.S. Pat. Nos. 4,100,055, 4,385,979, 4,457,825 and 4,657,654. When a sputter source of this type is at ambient temperature the target is slightly smaller in diameter than the cooling wall, and the target moves freely in an axial direction. This makes target replacement much easier than in the types of sputter sources described in the preceding paragraph. However, when the source is in operation and heats, the target expands into good physical and thermal contact with the cooling wall.
For a variety of reasons, sputter sources are often operated in a vertical or inverted orientation. To operate in such postures, sputter sources which rely on target expansion to provide good physical contact with a cooling wall require an additional holding means to maintain the sputter target in its proper position when the source is at ambient temperature. In the past, the additional holding means utilized have involved structures which must be screwed and unscrewed each time a target is changed. These prior art structures make sputter target changing a longer, more complex process than is desired.
One prior art solution to address the length of time it takes to change targets is shown in U.S. Pat. No. 4,657,654 in which the additional target holding means comprises pins, mounted at the outer rim of the target, which cooperate with holding slots. One disadvantage of this approach is the fact that target fabrication becomes more expensive and complex due to the need to securely mount the pins, which are made of a different material than the sputter target, into the target material. The pin mounting must remain secure notwithstanding repeated temperature cycling of the system.
Accordingly, it is an object of this invention to make it possible to quickly and easily change sputter targets in a sputter source of the type which relies on thermal expansion of an annular sputter target to provide physical contact with a peripheral cooling wall.
Another object of this invention is to provide a quick change sputter target and source structure which minimizes the risk of contamination of the sputter system vacuum chamber.
Yet another object of this invention is to provide a quick change target and source structure which can be easily retrofitted into existing sputtering systems.
Still another object of this invention is to provide a quick change target structure which can be used in a sputter source of the type described in U.S. Pat. No. 4,457,825.